Electric power generated in a power plant or the like is generally transmitted through a high-voltage transmission line to a power demand site, and in the power demand site, distributed through a distribution line to a power demand place such as a plant, a building, or a home after a voltage is reduced in several stages as required. When the electric power is supplied, a transformer for reducing the voltage, a distribution board for distributing the electric power, and the like are used. The transformer, the distribution board, and the like use a device for receiving and distributing the electric power, a controlling device such as a switch, and the like in order to receive and distribute the electric power at a large capacity and a low voltage. In addition, the device for receiving and distributing the electric power, the controlling device, and the like use an insulated bus bar, in which a region other than an electrical connection portion is provided as an electrical insulation portion by being coated with a tubular resin (insulating resin material), or a conductive member called an insulated bus duct, in which a plurality of such insulated bus bars are stacked (for example, Patent Literature 1).
For the conductive member, a copper-based material formed of copper or a copper alloy is mainly used because the copper-based material exhibits excellent performance in conductivity, strength, processability, corrosion resistance, and the like. However, the copper-based material is heavy in weight owing to, for example, copper having a density of 8.95 g/cm3 (20° C.) as compared to an aluminum material formed of aluminum or an aluminum alloy (for example, pure aluminum has a density of 2.699 g/cm3 (20° C.)). For example, in applications demanding weight saving, such as the bus duct to be used as a construction material, the aluminum material, which has a light weight and excellent conductivity, has begun to be used.
However, the aluminum material has the following problems. The aluminum material has a property of being easily oxidized in its surface, and hence when a conductive member formed of the aluminum material (aluminum conductive member) is exposed to external air, its surface is oxidized and an oxide film is easily formed, with the result that contact electrical resistance of the aluminum conductive member is increased owing to the oxide film, and electrical connection to a terminal to be connected is difficult to realize. Besides, when the aluminum conductive member is directly connected to a conductive member having a large difference in standard electrode potential, such as a conductive member formed of the copper-based material, electrical corrosion (electrochemical corrosion) occurs at the contact portion.
In such circumstances, a proposal for solving the problems of the aluminum conductive member has also hitherto been made. For example, in Patent Literature 2, there is a proposal of a plating method for imparting satisfactory conductivity and satisfactory rust resistance to a bus bar (aluminum bus bar) to be used in a bus duct. However, in such plating method, in which conductivity and rust resistance are imparted to the aluminum bus bar, plating is performed also on a region of the electrical insulation portion other than the electrical connection portion, which does not need conductivity. This disadvantageously entails a higher cost as the aluminum bus bar or the bus duct using the aluminum bus bar has a larger size. In addition, in Patent Literature 2, there is no disclosure of a method of forming the electrical insulation portion in a region other than the electrical connection portion, which is required in the case of using the aluminum bus bar as an insulated bus bar or an insulated bus duct. If the electrical insulation portion is formed with an insulating coating using a tubular resin or the like, there is a problem in that its long-term durability, chemical resistance, and the like depend on the resin in the electrical insulation portion.
It should be noted that, in Patent Literature 3, there is a disclosure of a housing made of an aluminum alloy for storing an electric vehicle secondary battery, the housing having on its surface a hard anodic oxide film having a thickness of from 20 μm to 100 μm and doubling as a bus bar. However, in Patent Literature 3, there is no disclosure of, for example, how the electrical connection portion to be used as a terminal is formed and how the conductivity and rust resistance of the formed electrical connection portion are ensured.